1. Field of the Invention
The present invention relates to a coating type magnetic recording medium for use in an audio tape, a video tape or the like, iron oxide magnetic powder contained in a magnetic layer of the magnetic recording medium and a manufacturing method therefor.
2. Description of Related Art
Hitherto, .gamma.-iron oxide coated with Co and thus having intense coercive force, that is, maghemite (.gamma.-Fe.sub.2 O.sub.3) has been employed to manufacture video tapes for use in home video tape recorders.
On the other hand, an intermediate of .gamma.-iron oxide coated with Co, that is, so-called magnetite (Fe.sub.3 O.sub.4), has been used widely because of a high magnetization rate and a high absorbance. Since magnetite has a large number of Fe.sup.2+ ions per unit volume thereof, it has a high magnetization rate. Therefore, it raises the magnetic flux density as a magnetic recording medium, thus causing the electromagnetic conversion characteristics to be improved.
However, the electromagnetic conversion characteristics depend upon the size of magnetic powder as well as the magnetic characteristics. Since the width of transition region for flux reversal is therefore reduced in proportion to the size of the magnetic powder, the high region (short wavelength) characteristics, which determine the resolution of an image, are affected considerably. As for the reproduction output, information can effectively be reproduced only in a case where the major axis of magnetic powder is not longer than 1/2 of the recording wavelength. The ratio (S/N) of an output signal with respect to noise which generates at the time of reproduction is considered to be in proportion to the square root of the inverse number of the volume of the magnetic powder.
To obtain the above-mentioned characteristics, the size of magnetic powder is required to be reduced. If the size of magnetic powder is reduced, the specific surface area (the surface area per unit weight) is enlarged and thus the cohesiveness among particles is intensified. Moreover, the number of particles per unit volume is enlarged. Therefore, the viscosity of the coating is raised, thus resulting in that magnetic powder cannot easily and uniformly be dispersed. That is, if magnetic powder is not uniformly dispersed, satisfactory surface and magnetic characteristics of the magnetic layer cannot be obtained. Thus, there arises a problem in that expected characteristics cannot be obtained.
However, the factor actually relating to magnetization of signals is the size of each crystal particle forming a single magnetic domain. Therefore, reduction in the size of crystallite contained in skeleton particles relating to dispersion while retaining the size of the skeleton particles enables the above-mentioned contrary characteristics to be realized.